1. Field of the Invention
The invention concerns a Faraday effect current sensor that has a coil of optical fiber through which is passed a conductor carrying current to be measured.
2. Description of the Related Art
Day et al.: "Faraday Effect Sensors: The State of the Art" Proc. SPIE, Vol. 985, pp 1-13 (1988) points out:
"Faraday effect current sensors are now used routinely in the measurement of large current pulses, and are starting to become available for ac current measurements in the power industry... They have several advantages. They can be constructed completely from dielectric materials, which is very important when operating at high voltage or in the presence of substantial electromagnetic interference. They are faster than other types of optical sensors for magnetic fields.... The principal disadvantage of Faraday effect sensors is generally thought to be their lack of sensitivity. Recent work has shown dramatic improvement in sensitivity, however, and further gains can be foreseen" (p. 1).
After describing a Faraday effect current sensor that employs a coil of optical fiber, the Day publication points out (at page 4) that "the primary difficulty with fiber current sensors is that linear birefringence in the fiber ... can seriously distort the response function" and (at page 5): "One technique for overcoming induced linear birefringence is to twist the fiber" to induce circular birefringence. Among other techniques discussed in the Day publication are to anneal the coil of optical fiber or to employ an optical fiber having high inherent circular birefringence.
U.S. Pat No. 4,255,018 (Ulrich et al.) concerns a Faraday effect current sensor wherein a conductor passes through a coil of optical fiber. Light from a polarizer is focused into the coiled optical fiber at its input end, and light that exits from the optical fiber is collimated and transmitted to an optical polarization analyzer, a photoelectric detector system, and a measuring instrument which together can be called a "polarization measuring device." The Ulrich invention is to twist the coiled optical fiber to produce a degree of "circular double refraction" that compensates for or suppresses, the "linear double refraction," those terms being synonymous with "circular birefringence" and "linear birefringence," respectively.
Ulrich's Faraday effect current sensor is similar to that of U.S. Pat No. 3,605,013 (Yoshikawa) which suggests that the "light" (polarization) axes of the polarizer and optical polarization analyzer should form an angle of 45.degree. and that the analyzer "is effective to derive a rotatory polarization component produced in proportion to the current" in an electrical conductor (col. 2, lines 34-37).
Chatrefou et al.: "Faraday Effect Current Sensor. Design of a Prototype" (a paper presented at the Workshop on the Role of Optical Sensors in Power Systems Voltage and Current Measurements, Gaithersburg, MD, Sept. 16-18, 1987) shows a Faraday effect current sensor much like that of Yoshikawa except that the light is reflected back through the coil to a semi-transparent mirror before reaching an analyzer "By having the light do a forward-return travel, by reflection at the fiber end (see FIG. 1), Faraday rotation is doubled, whereas rotation due to optical activity cancels out" (p 3).
Laming et al.: "Compact Optical Fibre Current Monitor with Passive Temperature Stabilization" (a paper presented at the Optical Fiber Sensors Topical Meeting of Optical Society of America, Washington, DC, Jan. 27-29, 1988) shows a Faraday effect current sensor much like that of the Chatrefou publication. In the Laming sensor, the light passes through collimators and a beam splitter both before and after passing through a 45.degree. polarizer. Then light transmitted by the coil passes through a third collimator and is reflected by a mirror back through the third and second collimators to the beam-splitter and on to a polarizing beam-splitter. By passing through so many interfaces, the light is significantly attenuated.
Another Faraday effect current sensor in which the light is reflected back through the coil is described in U.K. Pat. Appln. GB 2,104,213A, published 2 Mar. 1983.
Additional Faraday effect current sensors that employ optical fibers are described in U.S. Pat. No. 4,070,620 (Feldtkeller et al.); U.S. Pat. No. 3,590,374 (Evans et al.); and U.S. Pat. No. 3,419,802 (Pelenc et al.).
Other Prior Art
Single-mode optical fibers are known that propagate only one polarization state of the fundamental mode and so can be employed as polarizers. See, for example, U.S. Pat. No. 4,515,436 (Howard et al.) which describes a single-polarization optical fiber that preferably is stressed by being coiled or otherwise bent in order to have a broader bandwidth. Other single-polarization optical fibers are described in Simpson et al.: "A Single-Polarization Fiber" J. of Lightwave Tech, Vol LT-1, No. 2, pp 370-373 (1983); Simpson et al.: "Properties of Rectangular Polarizing and Polarization Maintaining Fiber" Proc. SPIE, Vol. 719, pp 220-225 (1986); Stolen et al.: "Short W-Tunneling Fibre Polarizers" Elec. Lett., Vol. 24, pp 524-525 (1988); Okamoto et al.: "High-Birefringence Polarizing Fiber with Flat Cladding" J. of Lightwave Tech, Vol. LT-3, No. 4, pp 758-762 (1985); Onstott et al.; "Polarization Controlling Optical Fibers," SPIE, Vol. 719, Fiber Optic Gyros: 10 th Anniversary Conference, Bellingham, WA; and Messerly et al.: "Broadband Single Polarization Optical Fiber," Optical Fiber Communication Conference, 22-26 Jan. 1990, San Francisco, CA. A single-polarization optical fiber is below called a "polarizing fiber."